Generally, a filter is a device for holding back or trapping some unexpected substances by use of its filtration passages. However, the filtration passages of the filter or the inlets of the filtration passages might be frequently clogged by relatively large particles or pieces (including substances to be filtered away) during a filtration process, thereby deteriorating or even completely destroying filtration function of the filter. Thus, it is very important on how to reduce or avoid clogging of the filtration passages of filters. It is not wise to simply increase cross-sectional areas of the filtration passages, since the filtration performance of the filter will be degraded as the cross-sectional areas increase.